This invention relates to radio frequency antenna arrays and, more particularly, the invention relates to calibrating radio frequency transceiver systems including antenna arrays to maximize their performance and effectiveness.
Antenna arrays can be used in any type of system that transmits or receives radio frequency signals using an antenna or antennas. Examples of such systems are radio communication systems, radars, and certain medical systems that employ radio frequency signals. The use of antenna arrays in such systems provides for antenna performance improvements over the use of a single element antenna, including improved directionality, signal to noise ratio, and interference rejection for received signals, as well as improved directionality, security, and reduced power requirements for transmitted signals. Antenna arrays can be used for signal reception only, for signal transmission only, or for both signal reception and transmission. Most antenna array systems consist of an antenna array and a signal processor that processes the signals going to and coming from the individual array elements.
Originally, manufacturers of antenna array systems used signal processors that assumed ideal antenna arrays. Great care was taken in designing and manufacturing the antenna arrays to ensure that the signal characteristics of the array were as close to ideal as possible. As a result, these antenna arrays were very difficult and expensive to manufacture.
Modern antenna array systems include a provision for storing and using calibration vectors that document the actual signal characteristics of the array. The signal processors for these systems use the calibration vectors to compensate for performance variances of the actual signal characteristics of the array. Unfortunately, conventional methods for measuring array calibration vectors have many drawbacks, including the requirement for extensive external measuring equipment which is both unwieldy and expensive. Further, conventional calibration methods are sensitive to drifts in system parameters such as frequency references over the extended period of time during which measurements are being made, and these drifts lead to inaccuracies in the measured antenna calibration vector.
Consequently, many antenna arrays are assigned generic calibration vectors for their particular design. Other antenna arrays are calibrated only once in the factory or upon initial installation. Conventional array calibration methods are sufficiently time consuming that it is impractical to calibrate an array at multiple angles or on a routine basis. Accordingly, antenna arrays that have not been individually and recently calibrated in their current environment will have inaccuracies in their array calibration vectors which result in performance degradation.